1. Field of the Invention
The present invention relates to an information recording medium in which information is recorded and played back with irradiation of a laser beam and more particularly to a double-layer Digital Versatile Disc (DVD) having two write-once information recording layers.
2. Discussion of the Background
In addition to read-only optical information recording media such as DVD-ROMs, recordable Digital Versatile Discs (DVD) such as DVD+RWs, DVD+Rs, DVD-Rs, DVD-RWs and DVD-RAMs have been introduced into the market. DVD+Rs and DVD+RWs are technically positioned as an extension of conventional recordable compact discs such as CD-Rs and CD-RWs. To secure compatibility between DVD+Rs and DVD+RWs and read-only DVDs, the recording density (track pitch and signal mark length) and the thickness of the substrate of DVD+Rs and DVD+RWs are changed to DVDs from CDs. The structure of a DVD+R and a CD-R is similar in the following ways: a dye is spin-coated onto a substrate to provide an optical information recording layer; further, a metal reflective layer is provided to the backside of the optical information recording layer to form an information recording substrate; and furthermore, another similar information recording substrate is attached to the information recording substrate with an adhesive material. In this case, a dye material is used to form the optical information recording layer. CD-Rs are characteristic in having a high reflectivity (i.e., 65%) complying with the specification of CDs. To obtain such a high reflectivity for the structure mentioned above, it is required that the optical information recording (absorbing) layer satisfies a predetermined complex refractive index for its recording wavelength and/or playback wavelength. The optical absorption characteristics of such a dye material is suitable to satisfy the requirement mentioned above. This is true in the case of DVD+Rs.
To increase the recording capacity, a read-only DVD having double information recording layers having improved optical absorption and reflection is proposed. FIG. 1 is a cross section of a DVD of prior art having double information recording layers. A substrate 1 and a substrate 2 are attached with a transparent intermediate layer 5 therebetween. The transparent intermediate layer 5 is formed of an ultraviolet curing resin. On the inside of the substrate 1, convexoconcave layers are formed, including a semi-transmittance layer 3 functioning as a first information recording layer is provided. On the inside of the substrate 2, a reflection layer 4 is formed. The semi-transmittance layer 3 is formed of a dielectric film or a thin metal film. The reflective layer 4 also functions as a second information recording layer and is formed of a metal layer or the like.
Recording signals recorded in each information recording layer are played back by using the effects of reflection and interference of a playback laser beam. By reading signals from the two information recording layers, its storage capacity becomes approximately 8.5 GB at maximum. Each of the substrate 1 and the substrate 2 has a thickness of 0.6 mm. The transparent intermediate layer 5 has a thickness of approximately 50 μm. The semi-transmittance layer 3, i.e., the first information recording layer, is formed to have a reflectivity of approximately 30%. The laser beam irradiated to play back information recorded in the reflective layer 4, also functioning as the second information recording layer, is reflected at the semi-transmittance layer 3 and attenuated in an amount of approximately 30% of the entire laser beam. Thereafter the rest of the laser beam is reflected at the reflective layer 4 layer functioning as the second information recording layer and attenuated again at the semi-transmittance layer 3 and goes out of the disc. A playback laser beam is focused on the first information recording layer or the second information recording layer to playback signals therein by detecting its reflection light. The wavelength of the laser beam for use in recording and/or playback signals in a DVD is approximately 650 nm.
However, the recordable DVDs, i.e., DVD+Rs, DVD-Rs, DVD-RW, DVD+RWs, etc., of prior art have only a single information recording layer readable from one side. To increase a storage capacity thereof, it is necessary to develop a medium which can be read from both sides. When an optical pickup irradiates with a writing laser beam the information recording layer located at the back in an optical information recording medium in which information is recorded and read in double information recording layers from one side with its focus thereon, there is a problem in that the other information recording layer located at the front therein attenuates the power of the laser beam so that the medium does not have a good combination of optical absorption necessary to record information in the second information recording layer and optical reflection therefrom. Published unexamined Japanese Patent Application No. (hereinafter referred to as JOP) 11-66622 discloses an optical information recording medium having double information recording layers formed of an organic dye in which information can be written and read from one side. However, this medium has just a structure of two attached substrates. These attached substrates have a conventional structure of a substrate on the incident side and a conventional structure of a substrate on the recording layer side. Therefore, the medium does not solve the problem of optical absorption and reflection deriving from the second information recording-layer mentioned above. In addition, there is no description about drawbacks in a semi-transmittance layer when the layer is thin. JOP 10-340483 discloses an optical information recording medium containing a metal reflective layer, a dye-containing recording layer and a protective layer. The medium can use SiO and SiO2 in the protective layer. However, there is no specific description about manufacturing conditions and optical characteristics. JOP 2000-311384 discloses an optical information recording medium having a second barrier layer between an intermediate layer (adhesive layer) and a second optical absorption layer. However, the literature describes only a single example in which Au is used as a material for the second barrier layer. When a metal is used as in this case, the absorption index k in the complex refractive index increases. As a result, it is impossible to fulfill the purpose of the present invention. Further, described in JOP 2000-311384, “absorption index k of a material for use in the second barrier layer is preferably not less than 0.1”. Thus there is no known teaching of lowering the absorption index k to a level not greater than 0.05.
For the foregoing reasons, a need exists for an optical information recording medium having double information recording layers to have a large storage capacity in which information can be recorded and played back from one side with good signal characteristics even for the recording layer located further from the light-incident side.